LSIII is a long neurotoxin from the venom of the blue-green sea snake Laticauda semifasciata. This sixty-six residue peptide binds to the nicotinic acetylcholine receptor in the post synaptic membranes of nerve and skeletal muscle with high selectivity and affinity, blocking neuromuscular response and inducing flaccid paralysis. We have recently determined the three dimensional structure of LSIII in solution using NMR. The main elements of secondary structure are a three-stranded anti-parallel beta-sheet and three finger-like loops protruding from a globular core, consistent with previously determined structures of other long-neurotoxins. The end of the most prominent loop is involved in receptor binding and is disordered relative to the rest of the molecule. We observed that this solvent-exposed loop has local order implying that this region moves as a semi-rigid body. The dynamics of this loop may play an important role in receptor binding. In last year's report, we described our progress in measuring heteronuclear relaxation parameters for the characterization of the internal dynamics of LSIII. Transverse and logitudinal relaxation rates were measured for 44 alpha-carbons with a precision of approximately 10%. The effective rotational correlation of LSIII estimated from this data was 4.6 ns, in good agreement with that of proteins of similiar size and with hydrodynamic calculations. This year, we completed our relaxation measurements at 600 MHz with the determination of """"""""C-'H NOEs for backbone alpha-carbons. The results were used to characterize the internal motions of LSIII using the approach of Lipari and Szabo. The parameters describing the dynamics were consistent with the results of molecular dynamics simulations, as well as the rigid-body motion of the binding loop inferred from the ensemble of structures. A manuscript describing this work is in preparation.
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